LTE IDLE MODE OPTIMIZATION IMPROVING

END USER EXPERIENCES

Kwangrok Chang, Ragil Putro Wicaksono, Seiji Kunishige, Noriteru Takagaki

MOTiV Research Co. Ltd., Tokyo Japan

{kwangrok.chang, ragil.wicaksono, seiji.kunishige, noriteru.takagaki}@motiv-research.com

Abstract— This paper presents the methodology of defining the

measurement trigger conditions and evaluation conditions of

LTE cell reselection for release 9 capable UEs. In the proposal of

cell reselection parameters, the RSRQ measurement quantity is

considered to account for the increase of packet traffic in LTE

networks. In addition, the proposal is designed to improve the

end user experiences such the LTE camp-on ratio, the avoidance

of unnecessary cell reselection measurements causing the battery

drain and the ping-pong effect between LTE and WCDMA inter-

RAT cell reselection or redirection. The cell reselection

parameter proposal is made based on the field test data collected

in Tokyo metropolitan area.

Keywords— LTE release 9, cell reselection, RSRQ, idle mode

parameters

I. INTRODUCTION

The UEs capable of 3GPP Release9 specification started

emerging in the mobile phone markets in the world from 2013

and eventually Apples’ iPhone5s/5c and other latest

smartphones have been also released stirring up lots of UE

subscribers’ desire to purchase the LTE UEs. Even if the

operators have not experienced high packet data traffic

brought by the smart phones yet and do not have

quantitatively measurable record to tell how much the packet

traffic will increase by the LTE UEs, they can down to earth

anticipate that the packet traffic volume in LTE RAN will

arise significantly and shortly, which will end up with serious

RSRQ deterioration in RF coverage perspective. In this case,

the Release8 idle mode parameters for determining UE’s cell

reselection behaviour [1], [2], which adopts only RSRP of the

serving cell and the target cells, won’t be good enough in the

high traffic network scenario and the operators will need the

provision of Release 9 idle mode parameters to accommodate

the RSRQ measurement quantity into idle mode cell

reselection [3]. It is inferable that some operators have already

revised their idle mode parameter settings in the preparation of

release 9 LTE UE launch.

II. LTE IDLE MODE OPTIMIZATION FOR END USER

EXPERIENCES

As in the WCDMA network where EcNo gets lower

resulted from the increase of own cell traffic and/or other cell

traffic, RSRQ in LTE network is also degraded when the

traffic in the own cell and/or other cell grows. If UE’s cell

reselection is performed only according to RSRP but not to

RSRQ, which is Release 8 version, UE won’t be able to

perform the cell reselection to a superior LTE target layer cells

even if the serving cell’s RSRQ went to a significantly low

level due to high traffic. As a result, the end user will

experience poor packet performance if RRC connection is

started.

RSRQ driven cell reselection becomes crucial in case there

is a possibility that the LTE traffic would not be equally

distributed over the multiple LTE layers. For example, if an

operator is running multiple LTE frequency bands where the

eNB’s antenna for each LTE frequency band is located in

different spots resulting in each eNB posing different RF

coverage footprints or even if the eNB for each frequency

band forms the similar RF coverage layout, RSRQs won’t be

the similar level in case the eNB vendors do not support traffic

load balancing feature over multiple LTE bands. Also if the

eNB vendors are different for each frequency band, the load

balancing between the multi-vendor are not generally

supported and as a result, RSRQ of the different frequency

bands could be far different from each other even if RSRP

levels are similar.

Cell reselection is performed according to the system

information parameters broadcasted in SIB3, SIB5, SIB6 of

LTE and SIB19 of WCDMA. The new parameters

additionally provided in Release 9 in the SIBs are summarized

TABLE 1 [3],[4].

TABLE 1. NEW LTE IDLE MODE PARAMETERS IN RELEASE 9

SIB Type Parameters

SIB3

s-IntraSearchP-r9

s-IntraSearchQ-r9

s-NonIntraSearchP-r9

s-NonIntraSearchQ-r9

q-QualMin-r9

threshServingLowQ-r9

SIB5

q-QualMin-r9

threshX-HighQ-r9

threshX-LowQ-r9

SIB6 threshX-HighQ-r9

threshX-LowQ-r9

SIB19

(WCDMA)

qualMinEUTRA

threshXhigh2

threshXlow2

It is aimed in this paper to propose the cell reselection

parameters taking into account the traffic load increase (RSRQ

measurement quantity) for LTE network. The target UE model

ISBN 978-89-968650-2-5 14 February 16~19, 2014 ICACT2014

of the new cell reselection parameters is the Release 9 capable

UEs. In the new cell reselection parameter proposal, the

RSRQ threshold for the measurement trigger and the

evaluation condition are investigated. The test area where the

new cell reselection parameters are developed is dense urban

areas in Tokyo with high packet traffic.

III. END USER’S IDLE MODE PERFORMANCE IN LTE

NETWORK

The LTE network operator’s objective for idle mode is to

have the LTE capable UEs hold the LTE coverage as long as

possible showing ‘4G’ or ‘LTE’ symbol on the mobile’s

screen. The assessment of how long or how high chance of

camping on the LTE cells rather than 3G cells can be

articulated by a performance indicator called ‘LTE camp-on

ratio’, which is the ratio of the LTE data samples against the

3G data samples collected by UE. The LTE camp-on ratio is

determined by the parameters relevant to the S-criteria and the

evaluation condition for LTE to WCDMA cell reselection.

Between the two conditions, the evaluation condition is crucial

to influence the LTE camp-on ratio as the evaluation condition

starts earlier than S-criteria by the value of .

As the 1st step of defining the UE’s measurement trigger for

the cell reselection, it is considered the end user idle mode

performance, which is interpreted as the number of LTE signal

bars displayed on the UE’s screen. Although the relation

between the number of signal bars and the packet call

performances may need to be studied separately however, in

general if the number of signal bars displayed is ‘3’ or more,

the LTE coverage can be considered acceptable or satisfying

level from end user’s perspective. Then, it is worthy to study

how the UE’s signal bars are determined upon LTE coverage

condition. iPhone5 was used for this investigation.

In order to study the relation between the number of the

signal bars and the actual RF coverage – RSRP and RSRQ, the

field test using iPhone5 and RF scanner was performed. The

RSRP and RSRQ measured by the scanner and the number of

signal bars data collected from the iPhone5 at the same time

stamp are compared for the same PCI that the iPhone5 was

camped on in idle mode.

Figure 1. RSRP PDF and CDF for signal bar 3

Figure 1. shows that RSRP = -90dBm is apparent threshold

by which the signal bar 3 is determined in iPhone5. It can be

said that serving cell’s RSRP shall be greater than or equal to -

90dBm to meet the end user’s RF coverage satisfaction, which

is signal bar 3 on screen.

(1)

IV. DEFINING RSRP AND RSRQ THRESHOLDS FOR

MEASUREMENT TRIGGER

UE’s measurement for intra-frequency and inter-frequency

cell reselection shall be triggered at least before reaching

RSRP = -90dBm. In general, in the areas nearby the serving

cell’s antenna site, the RSRP level must be good enough not

requiring the cell reselection to the neighboring cells and also

the neighbor cell’s RSRP condition is not at the comparable

level to the serving cell’s. At the cell edge or cell border areas,

the serving cell’s RSRP and RSRQ must be deteriorated due

to the propagation loss and noise and other cell interferences.

It can be re-written that the cell reselection measurement

trigger at the serving cell’s edge needs to be triggered with

high certainty that the serving cell’s RSRP will not fall below

-90dBm. The definition of serving cell edge is expressed as

follow in this paper.

(2)

It is noted from Figure 2. that the ratio of cell edge, defined

by 𝜟RSRP, against the overall data samples is slightly less

than 20% at RSRQ = -10dB. However, when the RSRQ is -

9dB or higher, the ratio of cell edge sample is negligible and

in adverse, the inner area samples where no suitable neighbor

cell for cell reselection can be found approaches almost 100%.

Figure 2. Ratio of various 𝜟RSRP for the Best cell’s RSRQ

It explains that if RSRQ = -9dB or higher value is used for

the measurement trigger for the cell reselection, nearly 100%

of locations within the cell’s coverage area is exposed to UE’s

measurement trigger even though there won’t be suitable

neighbor cells available for the cell reselection. The RSRQ, -

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

20%

RSRP Range (dBm)

Signal Bar 3 vs RSRP Range Distribution

PDF

CDF

0%

20%

40%

60%

80%

100%

-15 -14 -13 -12 -11 -10 -9 -8

RSRQ [dB]

Best Cell's RSRQ VS ΔRSRP Distribution

5 ≤ Δ < Max

4 ≤ Δ < 5

3 ≤ Δ < 4

2 ≤ Δ < 3

1 ≤ Δ < 2

0 ≤ Δ < 1

ISBN 978-89-968650-2-5 15 February 16~19, 2014 ICACT2014

9dB causes unnecessary cell reselection triggers to occur too

often and therefore, it is desirable to use -10dB for the RSRQ

threshold for Release 9 UE’s measurement trigger.

(3)

The next step is to define the measurement trigger threshold

of RSRP for release 9 UE. The lower bound of RSRP has been

defined as -90dBm. The upper bound for RSRP, shall be

determined in a way that UE can avoid the unnecessary

measurements for intra-frequency and inter-frequency

neighbors. The PDF distribution of RSRP and RSRQ

measured by RF scanner is presented in Figure 3. .

Figure 3. PDF of RSRP and RSRQ in a dense urban area LTE network

The ‘X-zone’, shown in Figure 3. , is the data samples

where the cell reselection measurement won’t be triggered for

the given RSRP and RSRQ thresholds. (e. g. RSRP and RSRQ

threshold was given as -84dBm and -10dB respectively)

From this network measurement data, it is calculated in

TABLE 2 how much ratio, the cell reselection measurement

will occur at the cell edge and inner areas within the cell’s

whole coverage when the different RSRP thresholds were

used. In this calculations, RSRQ threshold is fixed to -10dB.

TABLE 2. INTRA-FREQUENCY MEASUREMENT RATIO AT THE CELL EDGE

AND INNER AREAS FOR DIFFERENT RSRP

RSRP [dBm] Intra Ncell Measurement Area (inner area)

-80 75%

-84 65%

-88 60%

-92 55%

The higher the measurement ratio at the cell edge while the

lower measurement ratio in inner area, the better cell

reselection efficiency we aim to have. When RSRP = -84dBm

is selected for the trigger threshold, we can avoid unnecessary

measurements in the inner area while having 92% of cell edge

area be subject to the measurement trigger.

Therefore, it is recommended to set the thresholds of RSRP

for the release 9 UE’s intra-frequency cell reselection

measurement trigger as follow:

(4)

The cell reselection measurement for the inter-frequency

neighbors of which priority is equal to or lower than the

serving LTE layer, needs to be triggered marginally later than

the intra-frequency measurement by 2dB. Therefore it is

suggested to define the inter-frequency cell reselection

measurement as below:

(5)

V. DEFINING RSRP AND RSRQ THRESHOLDS FOR

EVALUATION CONDITION

When the target inter-frequency LTE layer or target inter-

RAT 3G layer is lower priority than the serving LTE layer,

there are three variants of cell reselection evaluation

conditions for the release 9 capable UE. For Release 9 UE

with threshServingLowQ provided:

(6)

, where is assumed to be ‘0’. (7)

For Release 9 UE without threshServingLowQ provided:

AND (8)

AND (9)

, where is assumed to be ‘0’.

If the inter-frequency LTE layer has the same priority than

the serving LTE layer does, the evaluation condition is given

as below:

(10)

It is known from all the evaluation conditions expressed

above that RSRQ measurement quantity is used for release 9

UE with threshServingLowQ parameter provided in SIB3.

Since it is aimed to develop an inter-frequency cell reselection

evaluation condition taking into account the traffic load not

only the RF coverage, RSRQ based evaluation condition needs

to be investigated.

The RSRQ evaluation threshold for the serving LTE cell,

‘ ’ defines the lower bound of

LTE coverage that if RSRQ gets worse than this value, UE

will perform inter-RAT cell reselection to WCDMA (assumed

that WCDMA coverage is always higher than the lower bound

-8

-9

-10

-11

-12

-13

-14

-15-68-72-76-80-84-88-92-96-100-104-108-112

RS

RQ

[d

B]

RSRP [dBm]

SBM RSRP, RSRQ Distribution at Cell Edge

4.5%-5.0%

4.0%-4.5%

3.5%-4.0%

3.0%-3.5%

2.5%-3.0%

2.0%-2.5%

1.5%-2.0%

1.0%-1.5%

0.5%-1.0%

0.0%-0.5%

X-zone

ISBN 978-89-968650-2-5 16 February 16~19, 2014 ICACT2014

of WCDMA coverage of evaluation condition). In order to

determine the RSRQ evaluation threshold, it is noted that there

is another lower bound of RSRQ in spite it is not in idle more

but in the RRC connected mode by which EUTRAN can

release the RRC connection by force and to redirect the UE to

WCDMA if RSRQ gets lower than this value. This parameter

in connected mode is called a2-threshold [5]. The parameter

value of a2-threshold provided in the

rrcConnectionReconfiguration message governs UE’s

measurement report of event A2.

When RSRQ is worse than a2-threshold due to high traffic

load or high interference, it is reckoned that LTE throughput

performance is no more superior to 3G’s so that the network

configures in EUTRAN to redirect the UE to 3G network. It

means that even if the evaluation threshold, ‘

’ for serving LTE cell is far lower than a2-

threshold, in other words, UE did not make cell reselection to

WCDMA even RSRQ is far lower than a2-threshold, the UE

will be redirected to 3G once the UE enters the RRC

connected mode. Therefore it is desirable to set the same

RSRQ value for the evaluation threshold, ‘

’ as a2-threshold value.

The next step is to determine the value of ,

which is used in the target cell’s evaluation condition. This

parameter is also used in both inter-frequency and inter-RAT

cell reselection evaluations. The parameter, is a

sort of hysteresis to avoid the ping-pong that UE comes back

to the old serving cell’s layer soon after the inter-frequency

LTE cell reselection was made. The parameter,

needs to be defined so as to be less sensitive to the

instantaneous fluctuation of RSRQ.

The stationary RSRQ measurement was performed at a

station of Tokyo city to grab how much RSRQ level fluctuates.

The measurement result is presented in RSRP and RSRQ

fluctuation range in urban stationary test.

Figure 4. RSRP and RSRQ fluctuation range in urban stationary test

It is revealed that 90% of RSRQ samples centred on the

median resides within 3.5dB fluctuation range. RSRP’s

fluctuation range is 5dB as reference information.

Taking into account 3dB RSRQ fluctuation range, the target

layer’s RSRQ shall be higher than serving layer’s RSRQ by

3dB or 4dB. 4dB of could be too high to satisfy

the evaluation condition for the target cell. Therefore, it is

recommended to use 3dB and as a result the parameter value

of is defined as:

(11)

By deciding the parameter values of

and , the evaluation condition utilizing RSRQ is

completed.

VI. CONCLUSIONS

The cell reselection parameters for release 9 capable UE

proposed in this paper can be various depending on the

operator’s strategy of LTE network deployment and the

absolute layer priorities between the LTE layers and WCDMA.

Nevertheless, the presented procedure and the methodology

used in this paper can be reused in any LTE networks with a

certain degree of customizations. Using the proposed values,

8% of unnecessary cell reselection measurement trigger could

be avoided and 22% of the end user perception of LTE

coverage during the idle mode could be improved in Tokyo

metropolitan area.

REFERENCE

[1] 3GPP TS36.304 v8.3.0, E-UTRA UE procedures in idle mode (Release 8)

[2] 3GPP TS36.133 v8.3.0, E-UTRA Requirements for support of radio resource management (Release 8)

[3] 3GPP TS36.304 v9.8.0, E-UTRA UE procedures in idle mode (Release 9)

[4] 3GPP TS25.304 v9.8.0, UE procedures in idle mode and procedures for cell reselection in connected mode (Release 9)

[5] 3GPP TS36.331 v9.8.0, E-UTRA Radio Resource Control (RRC) Protocol specification (Release 9)

[6] 3GPP TS25.304 v8.12.0, UE procedures in idle mode and procedures for

cell reselection in connected mode (Release 8)

[7] 3GPP TS25.133 v8.3.0, Requirements for support of radio resource management (FDD) (Release 8)

Kwangrok Chang (B.E.’93-M.E.’95-Ph.D.’98) received the degrees of Master and Ph.D in Electronic

and Electrical Engineering from POSTECH in 1995 and

1998, respectively. Since 2000, he has been with Nokia Siemens Networks Ltd. (NSN), where he led the

Network Planning and Optimization Group of Japan and Korea as the NPO head. He left NSN in 2011 and

co-founded MOTiV Research Co., Ltd with Seiji

Kunishige aiming for providing communication service providers and system vendors with the mobile network planning strategy and

optimization services and the development of the advanced network

performance enhancement methodology based on the big network data. His present interests are the enhancement of end user performances in high speed

data networks, such as HSPA and LTE and the innovation of network

performance data collection and its processing.

Ragil Putro Wicaksono (B.Eng’06-M.Eng’11) had

been working with NSN as network planning engineer for 2.5 years before received the degrees of Master in

Electronic and Electrical Engineering from Tokyo

Institute of Technology (東京工業大学 ), Japan, on

September, 2011. Afterwards he works in MOTiV

Research Co. Ltd., as a cellular network consultant. His

research interest are mainly related with mobility and

5%

5%

90% RSRP range: ~5dB

90% RSRQ range: ~3.5dB

UERSRP

UERSRQ

ISBN 978-89-968650-2-5 17 February 16~19, 2014 ICACT2014

radio resource management in cellular network (WCDMA, LTE, and LTE-A).

Seiji Kunishige (B.Eng’97) had worked at Couei as

customer service engineer for first launch of CDMA network(CDMA-one) in Japan for 4 years after

graduated Kansai University. He has worked at Nokia

Siemens Networks since 2001 and joined Network Planning and Optimization team. He led technical

consulting team of WCDMA/HSPA networks as

Solution Architect manager. He co-founded MOTiV Research Co., Ltd with Kwangrok Chang as CTO at

2011 and he is currently in charge of the general management of technical

issues.

Noriteru Takagaki had been with NSN for a decade mainly focusing on

network key performance indicator analysis and its data processing to identify the root causes of the abnormal network call performances. In 2011, he joined

MOTiV Research Co., Ltd. as the field test team manager maintaining the

quality of field tests and improving the efficiency of the network data collections.

ISBN 978-89-968650-2-5 18 February 16~19, 2014 ICACT2014